1. Field of the Invention
The present invention relates to the construction of a semiconductor device; and more particularly to the construction of walls that are impermeable to moisture (these walls being referred to hereinbelow as “seal rings”) that are provided as a single unit, that penetrate each of a plurality of interlayer dielectric films provided on a semiconductor substrate, and that are provided around the periphery of a semiconductor chip so as to enclose the inside portion of the semiconductor chip.
2. Description of the Related Art
In the process of fabricating a semiconductor device, after carrying out prescribed processes in the wafer state, the wafers are diced to chip size to obtain each semiconductor chip. A major issue in the fabrication of semiconductor devices is the prevention of moisture in the atmosphere or during dicing from permeating the cross section of the diced chips and thereby degrading the performance of the internal devices.
For this purpose, a method is known in which the outer periphery of a chip is covered by a metal pattern that is commonly referred to as a seal ring. The metal layer that functions as this seal ring is normally formed at the same time as the internal wiring patterns of the chip. In the prior art, this type of seal ring 2 is formed by a simple rectangular pattern that surrounds the outer perimeter of the chip, as shown in FIG. 1. Seal ring 2 is therefore formed with pointed shapes directed outward from the chip in the corners (as an example, refer to pages 3-5 and FIG. 3 of Japanese Patent Laid-Open Publication No. 2000-150429).
Typically, when the chip is exposed to a heat cycle, stress tends to concentrate in the corners of the chip, and the corners of the chip are therefore vulnerable to cracking. At this time, in a semiconductor chip in which seal ring 2 is formed in the above-described simple rectangular shape, there is a potential for cracking to occur in the corners in the areas inside seal ring 2. In such cases, moisture may enter the chip interior from the cracked portions, and the function of seal ring 2, which is to prevent the penetration of moisture, cannot be adequately achieved.
As a construction for suppressing the occurrence of this problem, one example is known in which seal ring 2 is arranged to extend at a 45° angle with respect to each side of the chip in the corners, as shown in FIG. 2. By means of this arrangement, seal ring 2 is positioned more toward the inside in the corners than in the rectangular pattern. As a result, this construction provides some effect, which is to allow a certain increase in resistance against moisture with regard to the intrusion of moisture into the chip interior that is caused by the occurrence of cracking in the corners of the chip.
Employing a plurality of seal rings similarly obtains the effect of suppressing the loss of the function of the seal ring due to the occurrence of cracking.
However, in the interest of extending the region of the chip that can be effectively used, the seal ring is preferably arranged as far toward the outside as possible. In a seal ring construction in which the seal ring is simply positioned inwardly in the corners as described above, the seal ring must also be arranged more toward the interior of the chip in areas other than the corners in order to prevent the effect of cracking in the corners. As a result, the area of the chip that can be effectively used is limited.
Further, a construction in which the number of seal rings is simply increased in some cases does not constitute an adequate countermeasure against cracking if the frequency of the occurrence of cracking in the corners and the depth of penetration are taken into consideration. In particular, semiconductor devices that employ interlayer dielectric films having a low dielectric constant that are being adopted in high integrated circuits in recent years call for an even stronger seal ring due to the degree of moisture absorption, the low degree of mechanical strength, and the low degree of adhesion to the wiring metal in the dielectric films themselves.